rebar3 fmt doesn't work with multiple files

[eproxus]

rebar3 fmt is documented to work with multiple files:

Usage: rebar3 fmt [-h] [-v] [-w] [-o <out>] [--verbose] [-c]
                  [--print-width <print_width>] [--require-pragma] [-i]
                  [--delete-pragma] [--range <range>]
                  [--exclude-files <exclude_files>] [<files>]

...

  <files>              files to format, - for stdin. If no files are
                       provided and one option of [-w, -o, -c] are
                       provided then
                       {src,include,test}/*.{hrl,erl,app.src} and
                       rebar.config are specified as the default.

However, when multiple files are supplied an error is printed:

$ rebar3 fmt file1.erl file2.erl
unknown option: {task,"file2.erl"}

(...usage text)